'Vain' in the Bible

And turn ye not aside: for then should ye go after vain things, which cannot profit nor deliver; for they are vain.

Then David strapped on his sword over his fighting attire, but he tried in vain to walk [around], for he was not trained to use [them]. So David said to Saul, "I am not able to walk with these, because I am not trained to use [them]." So David removed them.

Now David had said, Surely in vain have I kept all that this fellow hath in the wilderness, so that nothing was missed of all that pertained unto him: and he hath requited me evil for good.